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.TH SCTP 4P "Jul 30, 2009"
.SH NAME
sctp, SCTP \- Stream Control Transmission Protocol
.SH SYNOPSIS
.LP
.nf
#include <sys/socket.h>
#include <netinet/in.h>

s = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP);
.fi

.LP
.nf
\fBs = socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP);\fR
.fi

.LP
.nf
\fBs = socket(AF_INET6, SOCK_STREAM, IPPROTO_SCTP);\fR
.fi

.LP
.nf
\fBs = socket(AF_INET6, SOCK_SEQPACKET, IPPROTO_SCTP);\fR
.fi

.SH DESCRIPTION
.sp
.LP
SCTP is a transport protocol layered above the Internet Protocol (IP), or the
Internet Protocol Version 6 (IPv6). SCTP provides a reliable, session oriented,
flow-controlled, two-way transmission of data. It is a message- oriented
protocol and supports framing of individual messages boundaries. An SCTP
association is created between two endpoints for data transfer which is
maintained during the lifetime of the transfer. An SCTP association is setup
between two endpoints using a four-way handshake mechanism with the use of a
cookie to guard against some types of denial of service (DoS) attacks. These
endpoints may be represented by multiple IP addresses.
.sp
.LP
An SCTP message includes a common SCTP header followed by one or more chunks.
Included in the common header is a 32-bit field which contains the checksum
(computed using CRC-32c polynomial) of the entire SCTP packet.
.sp
.LP
SCTP transfers data payloads in the form of DATA chunks. Each DATA chunk
contains a Transmission Sequence Number (TSN), which governs the transmission
of messages and detection of loss. DATA chunk exchanges follow the Transmission
Control Protocol's (TCP) Selective ACK (SACK) mechanism. The receiver
acknowledges data by sending SACK chunks, which not only indicate the
cumulative TSN range received, but also non-cumulative TSNs received, implying
gaps in the received TSN sequence. SACKs are sent using the delayed
acknowledgment method similar to TCP, that is, one SCTP per every other
received packet with an upper bound on the delay (when there are gaps detected
the frequency is increased to one every received packet). Flow and congestion
control follow TCP algorithms: Slow Start, Congestion Avoidance, Fast Recovery
and Fast retransmit. But unlike TCP, SCTP does not support half-close
connection and "urgent" data.
.sp
.LP
SCTP is designed to support a number of functions that are critical for
telephony signalling transport, including multi-streaming. SCTP allows data to
be partitioned into multiple streams that have the property of independent
sequenced delivery so that message loss in any one stream only affects delivery
within that stream. In many applications (particularly telephony signalling),
it is only necessary to maintain sequencing of messages that affect some
resource. Other messages may be delivered without having to maintain overall
sequence integrity. A DATA chunk on an SCTP association contains the Stream
Id/Stream Sequence Number pair, in addition to the TSN, which is used for
sequenced delivery within a stream.
.sp
.LP
SCTP uses IP's host level addressing and adds its own per-host collection of
port addresses. The endpoints of an SCTP association are identified by the
combination of IP address(es) and an SCTP port number. By providing the ability
for an endpoint to have multiple IP addresses, SCTP supports multi-homing,
which makes an SCTP association more resilient in the presence of network
failures (assuming the network is constructed to provided redundancy). For a
multi-homed SCTP association, a single address is used as the primary address,
which is used as the destination address for normal DATA chunk transfers.
Retransmitted DATA chunks are sent over alternate address(es) to increase the
probability of reaching the  remote endpoint. Continued failure to send DATA
chunks over the primary address results in selecting an alternate address as
the primary address. Additionally, SCTP monitors the accessibility of all
alternate addresses by sending periodic "heartbeats" chunks. An SCTP
association supports multi-homing by exchanging the available list of addresses
during association setup (as part of its four-way handshake mechanism). An SCTP
endpoint is associated with a local address using the \fBbind\fR(3SOCKET) call.
Subsequently, the endpoint can be associated with additional addresses using
\fBsctp_bindx\fR(3SOCKET). By using a special value of \fBINADDR_ANY\fR with IP
or the unspecified address (all zeros) with IPv6 in the \fBbind()\fR or
\fBsctp_bindx()\fR calls, an endpoint can be bound to all available IP or IPv6
addresses on the system.
.sp
.LP
SCTP uses a three-way mechanism to allow graceful shutdown, where each endpoint
has confirmation of the DATA chunks received by the remote endpoint prior to
completion of the shutdown. An Abort is provided for error cases when an
immediate shutdown is needed.
.sp
.LP
Applications can access SCTP using the socket interface as a \fBSOCK_STREAM\fR
(one-to-one style) or \fBSOCK_SEQPACKET\fR (one-to-many style) socket type.
.sp
.LP
One-to-one style socket interface supports similar semantics as sockets for
connection oriented protocols, such as TCP. Thus, a passive socket is created
by calling the \fBlisten\fR(3SOCKET) function after binding the socket using
\fBbind()\fR. Associations to this passive socket can be received using
\fBaccept\fR(3SOCKET) function. Active sockets use the \fBconnect\fR(3SOCKET)
function after binding  to initiate an association. If an active socket is not
explicitly bound, an implicit binding is performed. If an application wants to
exchange data during the association setup phase, it should not call
\fBconnect()\fR, but use \fBsendto\fR(3SOCKET)/\fBsendmsg\fR(3SOCKET) to
implicitly initiate an association. Once an association has been established,
\fBread\fR(2) and \fBwrite\fR(2) can used to exchange data. Additionally,
\fBsend\fR(3SOCKET), \fBrecv\fR(3SOCKET), \fBsendto()\fR,
\fBrecvfrom\fR(3SOCKET), \fBsendmsg()\fR, and \fBrecvmsg\fR(3SOCKET) can be
used.
.sp
.LP
One-to-many socket interface supports similar semantics as sockets for
connection less protocols, such as UDP (however, unlike UDP, it does not
support broadcast or multicast communications).  A passive socket is created
using the \fBlisten()\fR function after binding the socket using \fBbind()\fR.
An \fBaccept()\fR call is not needed to receive associations to this passive
socket (in fact, an \fBaccept()\fR on a one-to-many socket will fail).
Associations are accepted automatically and notifications of new associations
are delivered in \fBrecvmsg()\fR provided notifications are enabled. Active
sockets after binding (implicitly or explicitly) need not call \fBconnect()\fR
to establish an association, implicit associations can be created using
\fBsendmsg()\fR/\fBrecvmsg()\fR or \fBsendto()\fR/\fBrecvfrom()\fR calls. Such
implicit associations cannot be created using \fBsend()\fR and \fBrecv()\fR
calls. On an SCTP socket (one-to-one or one-to-many), an association may be
established using \fBsendmsg()\fR. However, if an association already exists
for the destination address specified in the \fImsg_name\fR member of the
\fImsg\fR parameter, \fBsendmsg()\fR must include the association id in
\fImsg_iov\fR member of the \fImsg\fR parameter (using \fBsctp_sndrcvinfo\fR
structure) for a one-to-many SCTP socket. If the association id is not
provided, \fBsendmsg()\fR fails with \fBEADDRINUSE\fR. On a one-to-one socket
the destination information in the \fImsg\fR parameter is ignored for an
established association.
.sp
.LP
A one-to-one style association can be created from a one-to-many association by
branching it off using the \fBsctp_peeloff\fR(3SOCKET) call; \fBsend()\fR and
\fBrecv()\fR can be used on such peeled off associations. Calling
\fBclose\fR(2) on a one-to-many socket will gracefully shutdown all the
associations represented by that one-to-many socket.
.sp
.LP
The \fBsctp_sendmsg\fR(3SOCKET) and \fBsctp_recvmsg\fR(3SOCKET) functions can
be used to access advanced features provided by SCTP.
.sp
.LP
SCTP provides the following socket options which are set using
\fBsetsockopt\fR(3SOCKET) and read using \fBgetsockopt\fR(3SOCKET). The option
level is the protocol number for SCTP, available from
\fBgetprotobyname\fR(3SOCKET).
.sp
.ne 2
.na
\fB\fBSCTP_NODELAY\fR\fR
.ad
.sp .6
.RS 4n
Turn on/off any Nagle-like algorithm (similar to \fBTCP_NODELAY\fR).
.RE

.sp
.ne 2
.na
\fB\fBSO_RCVBUF\fR\fR
.ad
.sp .6
.RS 4n
Set the receive buffer.
.RE

.sp
.ne 2
.na
\fB\fBSO_SNDBUF\fR\fR
.ad
.sp .6
.RS 4n
Set the send buffer.
.RE

.sp
.ne 2
.na
\fB\fBSCTP_AUTOCLOSE\fR\fR
.ad
.sp .6
.RS 4n
For one-to-many style socket, automatically close any association that has been
idle for more than the specified number of seconds. A value of '0' indicates
that no associations should be closed automatically.
.RE

.sp
.ne 2
.na
\fB\fBSCTP_EVENTS\fR\fR
.ad
.sp .6
.RS 4n
Specify various notifications and ancillary data the user wants to receive.
.RE

.sp
.ne 2
.na
\fB\fBSCTP_STATUS\fR\fR
.ad
.sp .6
.RS 4n
Retrieve current status information about an SCTP association.
.RE

.sp
.ne 2
.na
\fB\fBSCTP_GET_ASSOC_STATS\fR\fR
.ad
.sp .6
.RS 4n
Gather and reset per endpoint association statistics.
.RE

.sp
.LP
Example Usage:
.sp
.in +2
.nf
#include <netinet/sctp.h>

struct sctp_assoc_stats stat;
int rc;

int32_t len = sizeof (stat);

/*
 * Per endpoint stats use the socket descriptor for sctp association.
 */

/* Gather per endpoint association statistics */
rc = getsockopt(sd, IPPROTO_SCTP, SCTP_GET_ASSOC_STATS, &stat, &len);
.fi
.in -2

.sp
.LP
Extract from the modified header file:
.sp
.in +2
.nf
sctp.h

 /*
  * SCTP socket option used to read per endpoint association statistics.
  */
  #define SCTP_GET_ASSOC_STATS          24

 /*
  * A socket user request reads local per endpoint association stats.
  * All stats are counts except sas_maxrto, which is the max value
  * since the last user request for stats on this endpoint.
  */
  typedef struct sctp_assoc_stats {
      uint64_t  sas_rtxchunks;   /* Retransmitted Chunks */
      uint64_t  sas_gapcnt;      /* Gap Acknowledgements Received */
      uint64_t  sas_maxrto;      /* Maximum Observed RTO this period */
      uint64_t  sas_outseqtsns;  /* TSN received > next expected */
      uint64_t  sas_osacks;      /* SACKs sent */
      uint64_t  sas_isacks;      /* SACKs received */
      uint64_t  sas_octrlchunks; /* Control chunks sent - no dups */
      uint64_t  sas_ictrlchunks; /* Control chunks received - no dups */
      uint64_t  sas_oodchunks;   /* Ordered data chunks sent */
      uint64_t  sas_iodchunks;   /* Ordered data chunks received */
      uint64_t  sas_ouodchunks;  /* Unordered data chunks sent */
      uint64_t  sas_iuodchunks;  /* Unordered data chunks received */
      uint64_t  sas_idupchunks;  /* Dups received (ordered+unordered) */
} sctp_assoc_stats_t;
.fi
.in -2

.SH MULTIHOMING
.sp
.LP
The ability of SCTP to use multiple addresses in an association can create
issues with some network utilities. This requires a system administrator to be
careful in setting up the system.
.sp
.LP
For example, the \fBtcpd\fR allows an administrator to use a simple form of
address/hostname access control. While \fBtcpd\fR can work with SCTP, the
access control part can have some problems. The \fBtcpd\fR access control is
only based on one of the addresses at association setup time. Once as
association is allowed, no more checking is performed. This means that during
the life time of the association, SCTP packets from different addresses of the
peer host can be received in the system. This may not be what the system
administrator wants as some of the peer's addresses are supposed to be blocked.
.sp
.LP
Another example is the use of IP Filter, which provides several functions such
as IP packet filtering (\fBipf\fR(8)) and NAT \fBipnat\fR(8)). For packet
filtering, one issue is that a filter policy can block packets from some of the
addresses of an association while allowing packets from other addresses to go
through. This can degrade SCTP's performance when failure occurs. There is a
more serious issue with IP address rewrite by NAT. At association setup time,
SCTP endpoints exchange IP addresses. But IP Filter is not aware of this. So
when NAT is done on a packet, it may change the address to an unacceptable one.
Thus the SCTP association setup may succeed but packets cannot go through
afterwards when a different IP address is used for the association.
.SH SEE ALSO
.sp
.LP
\fBipf\fR(8), \fBipnat\fR(8), \fBndd\fR(8),  \fBclose\fR(2),
.BR ioctl (2),
.BR read (2),
.BR write (2),
.BR libsctp (3LIB),
.BR accept (3SOCKET),
.BR bind (3SOCKET),
.BR connect (3SOCKET),
.BR getprotobyname (3SOCKET),
.BR getsockopt (3SOCKET),
.BR listen (3SOCKET),
.BR recv (3SOCKET),
.BR recvfrom (3SOCKET),
.BR recvmsg (3SOCKET),
.BR sctp_bindx (3SOCKET),
.BR sctp_freepaddrs (3SOCKET),
.BR sctp_getladdrs (3SOCKET),
.BR sctp_getpaddrs (3SOCKET),
.BR sctp_opt_info (3SOCKET),
.BR sctp_peeloff (3SOCKET),
.BR sctp_recvmsg (3SOCKET),
.BR sctp_sendmsg (3SOCKET),
.BR send (3SOCKET),
.BR sendmsg (3SOCKET),
.BR sendto (3SOCKET),
.BR socket (3SOCKET),
.BR inet (4P),
.BR inet6 (4P),
.BR ip (4P),
.BR ip6 (4P),
.BR tcp (4P),
.BR udp (4P),
.BR ipfilter (7)
.sp
.LP
R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, T. Taylor,  I.
Rytina, M. Kalla, L. Zang, V. Paxson, \fIRFC 2960, Stream Control Transmission
Protocol\fR, October 2000
.sp
.LP
L. Ong, J. Yoakum, \fIRFC 3286, An Introduction to Stream Control Transmission
Protocol (SCTP)\fR, May 2002
.sp
.LP
J. Stone, R. Stewart, D. Otis, \fIRFC 3309, Stream Control Transmission
Protocol (SCTP) Checksum Change\fR, September 2002.
.SH DIAGNOSTICS
.sp
.LP
A socket operation may fail if:
.sp
.ne 2
.na
\fB\fBEPROTONOSUPPORT\fR\fR
.ad
.RS 19n
The socket type is other than \fBSOCK_STREAM\fR and \fBSOCK_SEQPACKET\fR.
.RE

.sp
.ne 2
.na
\fB\fBETIMEDOUT\fR\fR
.ad
.RS 19n
An association was dropped due to excessive retransmissions.
.RE

.sp
.ne 2
.na
\fB\fBECONNREFUSED\fR\fR
.ad
.RS 19n
The remote peer refused establishing an association.
.RE

.sp
.ne 2
.na
\fB\fBEADDRINUSE\fR\fR
.ad
.RS 19n
A \fBbind()\fR operation was attempted on a socket with a network address/port
pair that has already been bound to another socket.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 19n
A \fBbind()\fR operation was attempted on a socket with an invalid network
address.
.RE

.sp
.ne 2
.na
\fB\fBEPERM\fR\fR
.ad
.RS 19n
A \fBbind()\fR operation was attempted on a socket with a "reserved" port
number and the effective user ID of the process was not the privileged user.
.RE

